Humidity and heat
The body retains more heat when the weather is hot and humid than it does during a drier but equally warm day. During excessive heat in the surrounding environment, our bodies perspire to make us feel cooler. Evaporation of the resulting perspiration works best when the air is dry. In moist, saturated air, perspiration cannot evaporate as readily. The combination of excess heat and moisture will cause us to feel hot and sticky. Generally speaking, the higher the humidity, the greater the discomfort.

The "humidex" system
The term "humidex" is a term used by Canadian meteorologists to describe how hot and humid weather feels to the average person. The humidex combines the temperature and humidity into one number to reflect the perceived temperature. While the term is widely used in Canada, extremely high readings are rare except in the southern regions of Ontario, Manitoba and Quebec. The hot, humid air masses causing high humidex readings usually originate in the Gulf of Mexico or the Caribbean.

Environment Canada has designed the following guide to comfort during humid weather:

Range of Humidex		Degree of Comfort
Less than 29°C		No discomfort
30° – 39°C		Some discomfort
40° – 45°C		Great discomfort; avoid exertion
Above 45°C		Dangerous
Above 54°C		Heat stroke imminent

When the humidex reads 40°C or more, all unnecessary outdoor activity should be curtailed.

If working outdoors is an absolute necessity, a reduced pace of activity, more frequent rest periods and drinking extra fluids are advised. On some occasions in Ontario, elevated smog levels and elevated humidex levels may exist at the same time. Use extreme caution during these periods!

Information provided by Environment Canada, Atmospheric Environment Service. For further information, visit Environment Canada's web site.